Head of MI5: Britain faces an unprecedented terror threat from Islamic State and al Qaeda fanatics, Andrew Parker (pictured) warned last night

Britain faces an unprecedented terror threat from Islamic State and Al Qaeda fanatics plotting ‘mass casualty’ attacks over the internet, the head of MI5 warned last night.

In a rare public speech, Andrew Parker said the Security Service thwarted six UK terror attacks in the past year and several overseas.

But he warned: ‘We can never be confident of stopping everything.’

The director-general of MI5 said the threat from jihadis was on a scale he had never seen in his 32-year career.

And he warned there was now a ‘greater ambition’ among terrorists to carry out mass casualty attacks in Britain as the threat showed ‘no sign of abating’.

‘It may not yet have reached the high water mark,’ he said.

In only his third speech since taking up the job in 2013, Mr Parker said terrorists were using a ‘bewildering array of devices and digital platforms’ to plot attacks, admitting MI5 had to carry out computer hacking against terror groups to access their communication networks.

He said internet firms also had an ‘ethical responsibility’ to help tackle terrorism.

Mr Parker said Britain was now facing a three-pronged threat – at home, overseas and online.

He identified ‘sophisticated exploitation’ of modern technology by IS – also known as Isil – to radicalise British teenagers was one major threat, but he warned Al Qaeda’s danger had ‘not gone away’.

The director-general said appalling acts had been ‘committed by individuals who were born and grew up here in the UK but for their own twisted reasons have decided to identify their own country as their enemy’.

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He revealed there were 3,000 suspected jihadis who were a ‘substantial challenge’ to the UK.

‘This year we have seen strong signs of greater ambition for mass casualty attacks by Isil,’ he said.

‘More than 750 people from this country have travelled to Syria to join extremist organisations and join in the fighting. The growth of the threat shows no sign of abating.’

Growing threat: In a rare public speech, Andrew Parker said the threat from homegrown and foreign jihadis was ‘on a scale and at a tempo’ that he had never before seen in his 32-year career (file picture)

SNOWDEN LEAK 'LIKE GIVING ENIGMA SECRETS TO NAZIS'

The leak of top secret material by Edward Snowden (pictured) was like telling the Nazis about the Enigma code-breaking project, Chris Inglis claims

Edward Snowden’s leak of top secret material was like telling the Nazis about the Enigma code-breaking project during the Second World War, a senior intelligence figure claims.

Chris Inglis, ex-deputy director of the US National Security Agency, said it was rebuilding its ability to track spies on the internet after Snowden ‘burnt the house down’ by leaking tens of thousands of classified files.

Snowden was a computer specialist at a US intelligence centre who tricked colleagues into revealing passwords so he could copy up to 1.7million files in one of the biggest leaks in US history.

He helped terrorists escape capture and assisted ‘rogue nations’ by revealing details of US and UK surveillance.

Mr Inglis compared the theft and leak of the documents to the impact on the war effort if the Nazis had been tipped off about the Allies’ knowledge of their Enigma machine, adding: ‘If we had made that plain and clear, we might have lost the battle of the North Atlantic. The effect is similar in this case.’

He said the Snowden leaks in 2013 ‘had a real and material’ impact on the fight against terrorism and the ability to spy on foreign countries, such as those involved in the proliferation of weapons of mass destruction, and international criminal gangs.

Mr Inglis told The Times: ‘We did lose some of our terrorist targets and see much better tradecraft on the part of various rogue nations and other actors.

‘Snowden may have burnt the house down but we’ll get a better house. I don’t give any credit to the arson,’ he said.

NSA deputy director Richard Ledgett has said the number of targets who disappeared off the radar following Snowden’s leaks was in the ‘high hundreds’.

Following his betrayal, Snowden fled the US to live in Moscow.

But he said technological change presented the UK with an ‘enormous challenge’ as it enabled terrorists to find places on the web that they could communicate without being ‘seen’.

And Mr Parker said US traitor Edward Snowden had given ‘our adversaries an advantage’ with his leaks about intelligence gathering.

He said the MI5’s ability to intercept communications had always been fundamental to its success, but this was becoming harder.

He said: ‘We are seeing plots against the UK directed by terrorists in Syria, enabled through contacts with terrorists in Syria and inspired online by Isil’s sophisticated exploitation of technology.

‘It uses the full range of modern communications tools to spread its message of hate, and to inspire extremists, sometimes as young as in their teens to conduct attacks in whatever way they can.

Shocking: In a recent sickening attack, a 19-year-old man is Syrian is run over by a tank by ISIS militants

Captive: The man, who was a Syrian army soldier, kneels in an orange jumpsuit as a jihadi stands behind him

‘The speed at which the process of radicalisation can occur online, and the emphasis on relatively low sophistication but nevertheless potentially deadly plots, are two major challenges that flow from Isil’s mode of operation.’

He was speaking ahead of the publication of two pieces of legislation – on combating extremism and giving security officials greater powers to monitor communications data – likely to prove hugely controversial.

The draft Investigatory Powers Bill is due to be published before Parliament rises for a week on November 10.

Home secretary: Theresa May (above) will study the result of a report on draft proposals on surveillance before placing a final bill before Parliament

After three major reports on surveillance, the bill will outline more explicitly than ever before the powers and capabilities of the intelligence services and the police in using surveillance to conduct investigations and gather information.

It will also propose a new style of regulation and scrutiny for bodies exercising those intrusive powers.

The draft proposals will be debated before a committee analyses them in detail and produces a report.

Theresa May, the home secretary, will study the result of the report before placing a final bill before Parliament.

Mr Parker said terrorists were using an array of devices and digital platforms, often provided by companies overseas.

He said an increasing proportion of these encrypted communications networks were ‘beyond our reach’.

And he said the UK was presented with an ‘enormous challenge’ with technological change which enabled terrorists to find places on the web that they could communicate without being ‘seen’.

‘Our ability to access and analyse data is more important than ever before’ he said.

‘We also need to draw upon a range of wider tools to mitigate the challenges.

‘This includes the ability to conduct operations online and to mount IT attacks, known as equipment interference, under a warrant authorised by the Home Secretary - against terrorist networks, so that we can access their communications.’

He said MI5 relied on the data to save lives and ‘without the tool we wouldn’t be able to keep the country safe.’